1. Technical Field
The present invention relates to an improvement of a steering control apparatus which electrically controls steering of a steering wheel by using an electric motor.
2. Prior Art
In recent years, a steering control apparatus is proposed in which a steering shaft coupled to a steering wheel is mechanically separated from a turning mechanism to turn a turning wheel, and the interlocking control of these is electrically performed by SBW (Steer By Wire) control (hereinafter referred to as SBW control).
In this steering control apparatus, an operation direction and a steering angle of the steering wheel are detected, a turning angle corresponding to the steering angle in the operation direction is given, and steering corresponding to the operation of the steering wheel is performed.
At that time, an electric motor is driven according to the operation amount of the steering wheel, turning force of a turning actuator, lateral force generated in a vehicle, yaw rate, and the like, and the steering force is transmitted to the steering shaft through a speed reducer.
Besides, the steering force includes, in addition to a normal steering auxiliary force, a steering reaction force to stop the rotation of the steering wheel in the case where the turning actuator is put in the maximum turning position.
In the SBW control, since the steering reaction force is generated by the electric motor, when the turning actuator is put in the maximum turning position, it has been difficult to prevent the rotation of the steering wheel by the electric motor.
For example, when a lock state occurs, a large output motor is required in order to generate the steering reaction force against the steering torque by the driver, and since a large current flows through the motor, there has been a fear that the motor is burnt by overheating. However, when current is limited in order to prevent the burning of the motor, a torque necessary for the steering assist motor is not obtained, and it has been impossible to restrict the excessive steering of the driver.
Then, in order to regulate the rotation of the steering wheel within a restriction range of a certain fixed allowable angle, some sort of mechanical braking means has been required.
Incidentally, in the case where the rotation of the steering wheel is made ±180 degrees or less, when a projection provided on a rotation shaft is brought into contact with a projection provided on a fixing member such as a housing, the rotation of the steering wheel can be regulated within a certain allowable rotation range. However, since the allowable rotation range of a normal steering wheel is approximately 3 to 4 turns, when the allowable rotation range is made ±180 degrees or less, a tire turning angle relative to a rotation angle of the steering wheel becomes excessively large. As a result, there has been a problem that the behavior of the vehicle becomes sensitive and steering stability is impaired.
Thus, as set forth in, for example, JP-A-2000-16316, means for setting a rotation limit angle of a steering wheel is disclosed in which a first projection provided at a steering shaft, a second projection provided at a steering shaft housing, a third projection and a fourth projection engageable with the first projection are provided to realize rotatable support around the steering shaft over 360 degrees or more.